Device for generating x-rays having a liquid metal anode

ABSTRACT

The invention relates to a device for generating X-rays ( 31 ). The device has a source ( 5 ) for emitting electrons ( 27 ) accommodated in a vacuum space ( 3 ). The X-rays are emitted by a liquid metal as a result of the incidence of the electrons. The liquid metal flows through a constriction ( 13 ) where the electrons emitted by the source impinge upon the liquid metal. The constriction is bounded by a thin window ( 23 ), which is made from a material which is transparent to electrons and X-rays and which separates the liquid metal in the constriction from the vacuum space, and by a wall ( 25 ) opposite to the window. According to the invention, the wall ( 25 ) has a profile (p) which matches a profile (p′) which the window ( 23 ) has,during operation, as a result of a deformation of the window caused by a pressure of the liquid metal in the constriction ( 13 ). Thus, it is achieved that the constriction has a predetermined intended cross-sectional area, and a decrease of the flow velocity and an accompanying excessive increase of the pressure at the location of the deformation of the window are prevented.

The invention relates to a device for generating X-rays, which devicecomprises a source for emitting electrons accommodated in a vacuumspace, a liquid metal for emitting X-rays as a result of the incidenceof electrons, and a pumping means for causing a flow of the liquid metalthrough a constriction where the electrons emitted by the source impingeupon the liquid metal, said constriction being bounded by a window,which is transparent to electrons and X-rays and separates theconstriction from the vacuum space, and by a wall opposite to thewindow.

A device for generating X-rays of the kind mentioned in the openingparagraph is known from U.S. Pat. No. 6,185,277-B1. The window of theknown device is relatively thin and is made from a material which istransparent to electrons and X-rays, e.g. diamond or molybdenum. Thewindow prevents the vacuum space from being contaminated by the liquidmetal. During operation of the known device, the liquid metal, e.g.mercury, flows through the constriction, which forms part of a closedchannel system. The source generates an electron beam, which passesthrough the window and impinges upon the liquid metal in an impingementposition in the constriction. The X-rays, emitted by the liquid metal asa result of the incidence of the electron beam, emanate through thewindow and through an X-ray exit window, which is provided in a housingenclosing the vacuum space. The velocity of the flow of the liquid metalin the constriction is relatively high, so that said flow is turbulent.As a result the heat, which is generated in the impingement position asa result of the incidence of the electron beam upon the liquid metal, istransported away from the impingement position by the flow of the liquidmetal in an effective manner. As a result, an increase of thetemperature of the liquid metal in the impingement position is limited,and a relatively high energy level of the electron beam is allowedwithout causing excessive heating of the liquid metal. The closedchannel system of the known device further comprises a heat exchanger bymeans of which the liquid metal is cooled down.

A disadvantage of the known device for generating X-rays is that therelatively thin window is deformed during operation as a result of apressure of the liquid metal in the constriction. As a result of thedeformation of the window, the cross-sectional area of the constrictionincreases at the location of the deformation. Said increasedcross-sectional area causes a reduction of the velocity of the flow ofthe liquid metal at the location of the deformation. As a result of theBernoulli effect, said reduction of the flow velocity causes an increaseof the pressure of the liquid metal at the location of the deformation,which pressure increase is comparatively high as a result of the highdensity of the liquid metal. As a result of said pressure increase, afurther deformation or even breakage of the thin window occurs.

It is an object of the invention to provide a device for generatingX-rays of the kind mentioned in the opening paragraph in which anincrease of the pressure of the liquid metal in the constriction as aresult of a deformation of the window is limited or even prevented, sothat the rate of deformation of the window is considerably reduced andbreakage of the window is prevented as much as possible.

In order to achieve said object, a device for generating X-raysaccording to the invention is characterized in that, at least duringoperation, said wall has a profile which matches a profile which thewindow has, during operation, as a result of a deformation of the windowcaused by a pressure of the liquid metal in the constriction.

The invention is based on the insight that a deformation of the windowunder the influence of the pressure of the liquid metal cannot beavoided, because the window should be relatively thin to obtainsufficient transparency to electrons and because a vacuum is present atone side of the window. Since, according to the invention, the wallopposite to the window has a profile which matches the profile which thewindow has, during operation, as a result of the deformation of thewindow, it is achieved that a cross-sectional area of the constrictionin the deformed state of the window, i.e. during operation,substantially corresponds with an intended, desired cross-sectionalarea, which the constriction would have if the window was not subject todeformation and if the wall did not have said profile. As a result, theflow velocity and hence the pressure of the liquid metal at the locationof the deformation substantially correspond with an intended flowvelocity and pressure, which the liquid metal would have if the windowwas not subject to deformation and if the wall did not have saidprofile. Accordingly, an increase of the pressure of the liquid metal atthe location of the deformation of the window is considerably limited oreven prevented, as a result of which the rate of deformation of thewindow and the risk of breakage of the window are considerably reduced.

It is noted that the expression “matches” in claim 1 is not meant to belimited to “is identical to” or “corresponds with”. Accordingly, theinvention does not only cover embodiments in which, during operation,the constriction has a constant cross-sectional area, seen in a flowdirection, but also embodiments in which, during operation, theconstriction has a cross-sectional area which changes in a predeterminedintended manner in the flow direction. Therefore, the expression“matches” generally intends to indicate that the profile of the wallopposite to the window is determined by, approximates, or correspondswith the profile of the deformed window in such a manner that thecross-sectional area of the constriction in the deformed state of thewindow, i.e. during operation, substantially corresponds with, andaccordingly also might change, seen in the flow direction, in a mannercorresponding with an intended cross-sectional area, which theconstriction would have if the window was not subject to deformation andif the wall did not have said profile.

A particular embodiment of a device according to the invention ischaracterized in that said wall is deformable by means of at least oneactuator, the device comprising at least one pressure sensor formeasuring the pressure of the liquid metal in the constriction and acontrol member for controlling the actuator as a function of a pressuremeasured by means of the sensor. In this embodiment, the actuator is forexample controlled in such a manner, and as a result the wall oppositeto the window is given such a profile, that the pressure measured by thesensor is maintained at a value corresponding with an intended pressure,which the liquid metal would have if the window was not subject todeformation and if the wall did not have said profile. Alternatively,the actuator is for example controlled in such a manner that thepressure of the liquid metal in the constriction does not exceed apredetermined safety value. Preferably, a plurality of sensors are used,so that the pressure can be measured in a plurality of locations in theconstriction, and a plurality of actuators are used, so that the profileof the wall opposite to the window can be adjusted in each locationwhere the pressure is measured.

A further embodiment of a device according to the invention ischaracterized in that said actuator is a piezo-electric actuator. Thepiezo-electric actuator is suitable for generating relatively small andaccurate deformations of the wall opposite to the window, so that theprofile of the wall can be adjusted very accurately. In addition, thepiezo-electric actuator can also be used as a pressure sensor, so thatthe structure of the device is considerably simplified.

A particular embodiment of a device according to the invention ischaracterized in that in the case of a deformation of the window, duringoperation, the constriction has a cross-sectional area which, seen in aflow direction, increases in such a manner that a reduction of the flowvelocity in the flow direction takes place such that a decrease of thepressure of the liquid metal in the constriction, caused by viscous flowlosses, substantially corresponds with an increase of said pressurecaused by said reduction of the flow velocity. If the constriction had aconstant cross-sectional area, seen in the flow direction, the liquidmetal would flow through the constriction at a velocity which issubstantially constant in the flow direction, and the pressure of theliquid metal would decrease, seen in the flow direction, as a result ofviscous flow losses. As a result, a relatively high pressure would benecessary at the entrance of the constriction in order to achieve acertain minimal pressure at the end of the constriction, which isnecessary to maintain a steady flow of the liquid metal throughout theconstriction. Said high pressure at the entrance of the constriction andthe accompanying pressure gradient between the entrance and the end ofthe constriction would cause a high mechanical load on the window, as aresult of which the risk of breakage of the window would stronglyincrease. In this particular embodiment, the profile of the wallopposite to the window is such that, in the case of a deformation of thewindow, during operation, the decrease of the pressure of the liquidmetal in the flow direction, caused by the viscous flow losses, issubstantially compensated by the increase of the pressure in the flowdirection, caused by the increase of the cross-sectional area and theaccompanying decrease of the flow velocity. Said increase of thepressure is a result of the Bernoulli effect. As a result, the pressureof the liquid metal is substantially constant throughout theconstriction and can be maintained at a relatively low level by asuitable design of the dimensions of the entire flow channel of thedevice and by a suitable flow rate of the liquid metal. As a result, themechanical load on the window is relatively low.

A particular embodiment of a device according to the invention ischaracterized in that the device is provided with a flow channel for theliquid metal which successively comprises, seen in a flow direction, aconverging part, said constriction, and a diverging part, wherein acenter line of at least a portion of said converging part, via which theconverging part is connected to the constriction, has a curvature whichmatches a curvature of a center line which the constriction has, duringoperation, in the case of a deformation of the window. In the case of adeformation of the window during operation, the constriction bounded bythe deformed window and by the profiled wall opposite to the window iscurved, seen in the flow direction. As a result, a curved flow ispresent in the constriction. An advantage of the curved flow is that theflow has a component in a direction transverse to the main flowdirection caused by centrifugal forces. As a result of said transversecomponent, the heat generated in the impingement position is moreeffectively distributed over the liquid metal flowing through theconstriction, so that the transfer of heat away from the impingementposition is improved. Since the center line of at least the portion ofthe converging part, via which the converging part is connected to theconstriction, has a curvature which matches the curvature of the centerline of the constriction, the curved flow in the constriction is alreadyinitiated in the converging part. As a result, the rate at which thecurved flow and in particular said transverse component will furtherdevelop in the constriction is considerably increased, so that the heattransfer away from the impingement position is further improved.

A particular embodiment of a device according to the invention ischaracterized in that the device is provided with a flow channel for theliquid metal which successively comprises, seen in a flow direction, aconverging part, said constriction, and a diverging part, wherein theconverging part is provided with means for generating or increasing aturbulence of the flow of the-liquid metal in the constriction. As aresult of said turbulence or increased turbulence of the flow of theliquid metal in the constriction, the heat generated in the impingementposition is more effectively distributed over the liquid metal flowingthrough the constriction, so that the transfer of heat away from theimpingement position is further improved.

A particular embodiment of a device according to the invention ischaracterized in that a center line, which the constriction has duringoperation as a result of said deformation of the window, is convex, seenfrom the source. In this embodiment, the window and the impingementposition, which is present at a relatively small distance below thewindow, are situated at an outer radius of the curved constriction. Atsaid outer radius, the local velocity in the main flow direction isrelatively high as a result of the fact that the liquid metal is forcedtowards said outer radius by centrifugal forces. As a result, thetransport of heat away from the impingement position is furtherimproved.

A particular embodiment of a device according to the invention ischaracterized in that the window is concave, seen from the source. Inthis embodiment, the window is situated at an inner radius of the curvedconstriction. Since the centrifugal forces, which are exerted on theflow of liquid metal in the curved constriction, are directed towardsthe outer radius of the curved constriction, i.e. away from the innerradius, the mechanical load on the window and the risk of breakage ofthe window are further reduced.

A further embodiment of a device according to the invention ischaracterized in that the window is provided with corrugations. As aresult of said corrugations, the mechanical strength of the window isimproved. In this manner, the window is better protected against damageor breakage, in particular when the device is started or stopped, inwhich cases the pressure of the liquid metal in the constriction canrise to values which are considerably higher than the value duringnormal operation.

A yet further embodiment of a device according to the invention ischaracterized in that said corrugations extend in a flow direction ofthe liquid metal in the constriction. In this manner, the corrugationsdo not lead to flow irregularities at the location of the window, sothat the transport of heat away from the impingement position isaffected hardly, or not at all by the presence of the corrugations.Furthermore, in this embodiment the increase of the flow losses in theconstriction as a result of the presence of the corrugations is limited.

A yet further embodiment of a device according to the invention ischaracterized in that the wall opposite to the window is provided withcorrugations which correspond with the corrugations of the window andare in positions, seen in a direction perpendicular to the flowdirection, identical to the positions of the corrugations of the window.In this manner, the local distances between the window and the wallopposite to the window, seen in a direction perpendicular to the flowdirection, are not influenced by the presence of the corrugations. As aresult, the presence of the corrugations does not influence thecross-sectional area of the constriction, and hence does not lead tolocal deviations of the main flow velocity and pressure of the liquidmetal.

In the following, embodiments of a device for generating X-raysaccording to the invention will be explained further in detail withreference to the Figures, in which

FIG. 1 schematically shows a first embodiment of a device for generatingX-rays according to the invention,

FIG. 2 shows a constriction of a device similar to the device of FIG. 1,but without a profiled wall bounding said constriction,

FIG. 3 shows, during operation, a constriction with a profiled wall ofthe device of FIG. 1,

FIG. 4 shows, during operation, a constriction with a profiled wall of asecond embodiment of a device for generating X-rays according to theinvention,

FIG. 5 shows, during operation, a constriction with a profiled wall of athird embodiment of a device for generating X-rays according to theinvention,

FIG. 6 shows, during operation, a converging part, a constriction, and adiverging part of a fourth embodiment of a device for generating X-raysaccording to the invention,

FIG. 7 shows, during operation, a converging part, a constriction, and adiverging part of a fifth embodiment of a device for generating X-raysaccording to the invention,

FIG. 8 shows a cross-section along the line VIII-VIII in FIG. 7, and

FIG. 9 shows the device of FIG. 7 provided with an alternative solutionto prevent buckling of the window bounding the constriction.

In FIG. 1 only the main components of the first embodiment of a devicefor generating X-rays according to the invention are schematicallyshown. The device comprises a housing 1 which encloses a vacuum space 3in which a source 5 or cathode for emitting electrons is accommodated.The device further comprises a closed channel system 7 comprising aninlet channel 9, a converging part 11, a constriction 13, a divergingpart 15, an outlet channel 17, a heat exchanger 19, and a hydraulic pump21. The channel system 7 is filled with a liquid metal which has theproperty of emitting X-rays as a result of the incidence of electrons.In the embodiment shown, the liquid metal is an alloy of Ga, In, and Sn,but also other kinds of metals or metal alloys which are liquid at roomtemperature, such as for example Hg, may be used. The constriction 13 isbounded by a window 23, which is transparent to electrons and X-rays,and by a wall 25 opposite to the window 23. In the embodiment shown, thewindow 23 comprises a relatively thin diamond plate, but also otherkinds of materials which are sufficiently transparent to electrons andX-rays, such as for example Mo, may be used. The window 23 separates theconstriction 13 from the vacuum space 3, thereby preventing the vacuumspace 3 from being contaminated by particles of the liquid metal.

During operation of the device, the liquid metal is caused to flowthrough the constriction 13 by means of the hydraulic pump 21. In theembodiment shown, the hydraulic pump 21 is of a conventional type, butalso another suitable pumping means may be used instead, such as forexample a magneto-hydrodynamic pump. The constriction 13 has arelatively small cross-sectional area, so that the flow of the liquidmetal in the constriction 13 has a relatively high velocity and isturbulent. The source 5 generates an electron beam 27, which passesthrough the window 23 and impinges upon the liquid metal in animpingement position 29 in the constriction 13. As a result of theincidence of the electron beam 27 upon the liquid metal, X-rays 31 aregenerated in the impingement position 29. Thus, the liquid metal in theconstriction 13 constitutes an anode of the device for generatingX-rays. The X-rays 31 emanate through the window 23 and through an X-rayexit window 33, which is provided in the housing 1.

A further result of the incidence of the electron beam 27 upon theliquid metal is the generation of a large amount of heat in theimpingement position 29. This heat is transported away from theimpingement position 29 in an effective manner by the flow of the liquidmetal in the constriction 13, and the heated liquid metal issubsequently cooled down again in the heat exchanger 19. In this manner,excessive heating of the liquid metal in the impingement position 29 andof the surroundings of the constriction 13 is prevented. By means of theflow of the liquid metal in the constriction 13, a relatively high rateof heat transport away from the impingement position 29 is achieved, sothat a relatively high energy level of the electron beam 27 andconsequently a relatively high energy level of the X-rays 31 is allowed.

In order to obtain a sufficiently high velocity of the liquid metal inthe constriction 13 during operation, the pump 21 generates a relativelyhigh pressure of the liquid metal. In the embodiment shown, a pressurein the order of 50-60 bar is generated in the inlet channel 9 to obtaina flow velocity in the order of 50 m/s in the constriction 13. In theembodiment shown, the constriction 13 has a height, i.e. a distancebetween the window 23 and the wall 25, of approximately 400 μm, a lengthin the flow direction of approximately 1.5 mm, and a width perpendicularto the flow direction of approximately 10 mm. As a result of theBernoulli effect in the converging part 11, the pressure in theconstriction 13 is in the order of 1 bar. As a result of the Bernoullieffect in the diverging part 15, the pressure in the outlet channel 17is in the order of 40-45 bar, which is lower than the pressure in theinlet channel 11 as a result of viscous flow losses.

Under the influence of the pressure of the liquid metal in theconstriction 13, the window 23 is deformed. A deformation of the window23 cannot be avoided, because the window 23 should be sufficiently thinto achieve sufficient transparency to electrons and X-rays, and becauseat the side of the window 23 remote from the liquid metal a vacuumpressure is present. In the embodiment of FIG. 1, a maximal deformationin the middle of the window 23 is in the order of 30 μm. If the wall 25′opposite to the window 23′ were straight, as shown in FIG. 2 whichschematically shows a constriction 13′ of a device not covered by theinvention, the deformation of the window 23′ would lead to an increaseof the cross-sectional area of the constriction 13′ at the location ofthe deformation. Said increase of the cross-sectional area would lead toa decrease of the flow velocity and, as a result of the Bernoullieffect, to an increase of the pressure at the location of thedeformation. Said increase of the pressure would be relatively high as aresult of the high density of the liquid metal. With a typical densityin the order of 8000 kg/m³, an increase of the pressure in the order of10 bar would be obtained at the location of the maximal deformation inthe middle of the window 23′, which is indicated by the reference d inFIG. 2. Such an increase of the pressure would lead to a furtherinadmissible deformation of the window 23′ or even to breakage of thewindow 23′.

In order to limit or even prevent an increase of the pressure of theliquid metal in the constriction 13 as a result of the deformation ofthe window 23, the wall 25 opposite to the window 23 has a profile p, asshown in FIG. 3, which corresponds with a profile p′ which the window 23has, during operation, as a result of the deformation of the window 23caused by the pressure of the liquid metal in the constriction 13. Inthe embodiment shown, the wall 25 is relatively thick and made from amaterial having a relatively low transparency to X-rays, so that theprofile p of the wall 25 is fixed. The profile p of the wall 25corresponds with a profile which the window 23 would have if thepressure exerted on the window 23 corresponded with the pressure in animaginary straight constriction, i.e. a constriction bounded by anundeformed straight window and by a straight wall opposite to thewindow. The profile p can be predetermined by means of, for example, anumerical calculation. In this way it is achieved that thecross-sectional area of the constriction 13 in the deformed state of thewindow 23 as shown in FIG. 3, i.e. during operation, substantiallycorresponds with the cross-sectional area of said imaginary straightconstriction, i.e. the cross-sectional area of the constriction 13 isconstant in the flow direction X. As a result, the flow velocity andhence the pressure of the liquid metal at the location of thedeformation of the window 23 substantially correspond with the flowvelocity and the pressure in said imaginary straight constriction, andaccordingly an increase of the pressure of the liquid metal at thelocation of the deformation of the window 23 is limited or even absent.

FIG. 4 shows a constriction 35 of a second embodiment of a device forgenerating X-rays according to the invention. Parts of the secondembodiment, which correspond with parts of the first embodiment as shownin FIGS. 1 and 3, are indicated by means of corresponding referencenumbers. Apart from the constriction 35, the second embodimentsubstantially corresponds with the first embodiment, and therefore theother parts of the second embodiment are not shown in the Figures andwill not be discussed. In the constriction 35, the wall 25 opposite tothe window 23 has a profile p₁ which does not substantially correspondwith the profile p₁′ which the window 23 has as a result of thedeformation of the window 23 caused by the pressure of the liquid metalin the constriction 35, but which matches the profile p₁′ of thedeformed window 23 in such a manner that a cross-sectional area of theconstriction 35 gradually increases in the flow direction X. Thus, across-sectional area A₁ at the entrance 37 of the constriction 35 issmaller than a cross-sectional area A₂ at the location of the maximaldeformation d in the middle of the window 23, and said cross-sectionalarea A₂ is smaller than a cross-sectional area A₃ at the end 39 of theconstriction 35. If the viscous flow losses in the constriction 35 werezero, the flow velocity would gradually decrease in the flow direction Xas a result of said increasing cross-sectional area and, as a result ofthe Bernoulli effect, the pressure of the liquid metal would graduallyincrease in the flow direction X. The profile p₁ of the wall 25 and as aresult, in the deformed state of the window 23, the increase of thecross-sectional area of the constriction 35 in the flow direction X aresuch that a decrease of the pressure of the liquid metal in the flowdirection X, caused by the viscous flow losses in the constriction 35,substantially corresponds with, and hence is substantially compensatedby, said increase of the pressure in the flow direction X caused by theBernoulli effect. As a result, the pressure of the liquid metal in theconstriction 35 is substantially constant in the flow direction X, sothat the window 23 is subjected to a uniform mechanical load. In thisembodiment, by a suitable design of the dimensions of the entire channelsystem 7 and by a suitable flow rate of the liquid metal, a relativelylow uniform pressure of less than 1 bar can be maintained duringoperation throughout the constriction 35, so that the mechanical load onthe window 23 is further reduced. This pressure corresponds with apressure in an imaginary constriction which is bounded by an undeformedwindow and by a wall opposite to the window which tapers relative to thewindow in the upstream direction.

FIG. 5 shows a constriction 41 of a third embodiment of a device forgenerating X-rays according to the invention. Parts of the thirdembodiment, which correspond with parts of the second embodiment asshown in FIG. 4, are indicated by means of corresponding referencenumbers. Apart from the constriction 41, the third embodimentsubstantially corresponds with the second embodiment, and therefore theother parts of the third embodiment are not shown in the Figures andwill not be discussed. In the constriction 41, the wall 43 opposite tothe window 23 does not have a fixed profile like the walls 25 in thefirst and second embodiments described before. The wall 43 is a surfaceof a relatively thin metal plate 45 with, in the embodiment shown, athickness of 200 μm. The plate 45 and accordingly also the wall 43 aredeformable in a direction transverse to the flow direction X by means ofa number of piezo-electric actuators 47, which are accommodated in aclosed chamber 49 below the plate 45. In an undeformed state, the wall43 has a profile p₂ which roughly corresponds with the profile p₁ of thewall 25 in the second embodiment. Thus, like the wall 25 in the secondembodiment, the profile p₂ matches the profile p₁′ of the deformedwindow 23 in such a manner that a decrease of the pressure of the liquidmetal in the flow direction X, caused by the viscous flow losses in theconstriction 41, roughly corresponds with and hence is roughlycompensated by, the increase of the pressure in the flow direction X,caused by the Bernoulli effect which is caused by the increasingcross-sectional area of the constriction 41 in the flow direction X.

The third embodiment further comprises a control member 51 whichcontrols the actuators 47 as a function of a pressure of the liquidmetal in the constriction 41 measured by means of a pressure sensor. Inthe embodiment shown, the piezo-electric actuators 47 are also used aspressure sensors, the actuators 47, periodically supplying electricalsignals u_(P,i), corresponding with a pressure exerted on the actuators47 to the control member 51, and the control member 51 periodicallysupplying electrical signals u_(D,i) corresponding with a deformation ofthe actuators 47 determined by the control member 51 in response to thesignals u_(P,i). The signals u_(D,i) are determined by the controlmember 51 to be such, and accordingly the wall 43 is deformed to havesuch a profile p₂′, that the pressure of the liquid metal in theconstriction 41, measured by each of the actuators 47, corresponds witha predetermined constant value below 1 bar. Thus, it is achieved thatthe pressure of the liquid metal in the constriction 41 is maintained atsaid predetermined value in a very accurate manner, particularly in caseof deviations of the pressure and of the velocity in the converging part11 and in case of deviations of the deformation of the window 23 causedby, for example, deviations of the temperature. The piezo-electricactuators 47 are suitable for generating relatively small and accuratedeformations of the wall 43, so that the profile p₂′ of the wall 43 canbe adjusted very accurately. In addition, the structure of the device isrelatively simple in that the actuators 47 also constitute the necessarypressure sensors. It is however noted that the invention also comprisesembodiments in which separate pressure sensors are used to measure thepressure of the liquid metal in the constriction 41, and/or in whichanother type of actuator is used. The invention also comprisesembodiments in which the structure of the device is further simplifiedin that fewer actuators and pressure sensors, or even only one actuatorand pressure sensor, for example only at the location of the maximaldeformation d in the middle of the window 23, are used. The inventionfurther comprises embodiments in which, instead of pressure sensors,sensors are used which measure the deformation of the window 23. In suchan embodiment, the actuators 47 are controlled in such a manner that,during operation, the deformation of the window 23 corresponds with apredetermined intended deformation.

In the embodiments of FIGS. 1, 3, 4, and 5 described before, the window23 is convex, seen from the source 5 and from the electron beam 27, as aresult of the deformation of the window 23 during operation. Since, inthese embodiments, the profile p, p₁, p₂′ of the wall 25,43 opposite tothe window 23 corresponds with or matches the profile p′, p₁′ of thedeformed window 23, the profile p, p₁, p₂′ of the wall 25, 43 and acenter line, which the constriction 13, 35, 41 has as a result of thedeformation of the window 23, are also convex, seen form the source 5.As a result, the flow of the liquid metal in the constriction 13, 35, 41is curved during operation, the window 23 being present near an outerradius R_(O) of the curved flow, and the wall 25 being present near aninner radius R_(I) of the curved flow as shown in FIG. 3. In the curvedflow a centrifugal force is exerted on the liquid metal, which urges theliquid metal towards the outer radius R_(O) and accordingly causes aflow component in a direction transverse to the main flow direction X.As a result, as shown in FIG. 3, the local velocity V_(L) in the mainflow direction X is relatively high near the outer radius R_(O). Sincethe impingement position 29 is present at a relatively small distancebelow the deformed window 23, i.e. near the outer radius R_(O) where thelocal velocity V_(L) is relatively high, the transport of heat away fromthe impingement position 29 is considerably improved as a result of theconvex, curved flow.

FIG. 6 shows a converging part 53, a constriction 13, and a divergingpart 55 of a fourth embodiment of a device for generating X-raysaccording to the invention. Parts of the fourth embodiment, whichcorrespond with parts of the first embodiment as shown in FIGS. 1 and 3,are indicated by means of corresponding reference numbers. Apart fromthe converging part 53 and the diverging part 55, the fourth embodimentsubstantially corresponds with the first embodiment, and therefore theother parts of the fourth embodiment are not shown in the Figures andwill not be discussed. The constriction 13 of the fourth embodimentcorresponds with the constriction 13 of the first embodiment shown inFIG. 3, but another constriction within the scope of the invention, suchas the constriction 35 of the second embodiment or the constriction 41of the third embodiment, may be used instead. FIG. 6 further shows thecenter line 57 of the constriction 13, which is convex, as seen from thesource 5 and the electron beam 27, as a result of the convex deformationof the window 23 during operation. In the fourth embodiment, theconverging part 53 has a portion 59 via which the converging part 53 isconnected to the constriction 13. As shown in FIG. 6, said portion 59has a center line 61 with a curvature which matches the curvature of thecenter line 57, which the constriction 13 has, during operation, in thecase of a deformation of the window 23. As a result, a curved flow,having a curvature corresponding with the curvature of the curved flowin the constriction 13, is also present in the portion 59 of theconverging part 53. In this manner, the curved flow in the constriction13 is already initiated in the portion 59 of the converging part 53. Asa result, the flow component in the direction transverse to the mainflow direction X will already arise in the portion 59 of the convergingpart 53 and will further increase in the constriction 13, so that thelocal velocity V_(L) in the constriction 13 near the outer radius R_(O)and accordingly also the rate of heat transfer away from the impingementposition 29 will further increase.

In the fourth embodiment shown in FIG. 6, the diverging part 55 has aportion 63 via which the constriction 13 is connected to the divergingpart 55. Said portion 63 has a center line 65 with a curvature which,like the center line 61 of the portion 59 of the converging part 53,matches the center line 57 of the constriction 13. As a result, thecurved flow in the constriction 13 is maintained partially in saidportion 63 of the diverging part 55. As a result, the curved flow in theconstriction 13 is substantially not affected by the presence of thediverging part 55. It is noted, however, that the invention also coversembodiments in which only the curvature of the center line 61 of theportion 59 of the converging part 53 matches the curvature of the centerline 57 of the constriction 13. In such an embodiment, the curved flownear the end of the constriction 13 might be slightly affected by thepresence of the diverging part 55, but the advantages of a curved floware still achieved in a relatively large portion of the constriction 13.

As discussed before, the flow velocity of the liquid metal in theconstriction 13, 35, 41 has such a value that the flow of the liquidmetal in the constriction 13, 35, 41 is turbulent. As a result of theturbulence, the heat generated in the impingement position 29 as aresult of the incidence of electrons is effectively distributed over theliquid metal flowing through the constriction 13, 35, 41, particularlyin a direction transverse to the main flow direction X, so that thetransfer of heat away from the impingement position 29 is improved. Inthe fourth embodiment shown in FIG. 6, the converging part 53, andparticularly the portion 59 of the converging part 53, is provided withmeans for increasing the turbulence of the flow of liquid metal in theconstriction 13, so that the transfer of heat away from the impingementposition 29 is further improved. In the embodiment shown, said means forincreasing the turbulence comprise a number of rods 67 which arearranged at regular interspaces in the converging part 53 and in theportion 59. It is noted that instead of said rods 67 other suitablemeans for increasing the turbulence may be used. Such means can also beused in the other embodiments of the invention shown in FIGS. 1, 3, 4,and 5. In the embodiment of FIG. 6, the distribution of the heat in thedirection transverse to the main flow direction X is further improved bythe presence of so-called Taylor-Goertler vortices in the constriction13, which are characteristic of a curved flow. Since the curved flow isalready present in the portion 59 of the converging part 53, saidvortices will already arise in the portion 59 and will further increasein the constriction 13. In the embodiment shown, the interspaces betweenthe rods 67 substantially correspond with the period of saidTaylor-Goertler vortices during operation. In this manner, the rods 67do not only increase the turbulence of the flow of liquid metal in theconstriction 13, but also increase the Taylor-Goertler vortices in theconstriction 13. As a result, the distribution of the heat in thedirection transverse to the main flow direction X is further improved.

FIG. 7 shows a converging part 69, a constriction 71, and a divergingpart 73 of a fifth embodiment of a device for generating X-raysaccording to the invention. Parts of the fifth embodiment, whichcorrespond with parts of the first embodiment as shown in FIGS. 1 and 3,are indicated by means of corresponding reference numbers. Apart fromthe converging part 69, the constriction 71, and the diverging part 73,the fifth embodiment substantially corresponds with the firstembodiment, and therefore the other parts of the fifth embodiment arenot shown in the Figures and will not be discussed here. In the fifthembodiment, the constriction 71 is bounded by a window 75 which isconcave, seen from the source 5, both in a state in which the window 75is not deformed and in a state in which the window 75 is deformed duringoperation by the pressure of the liquid metal in the constriction 71.The constriction 71 is further bounded by a wall 77 opposite to thewindow 75. Said wall 77 has a fixed profile p₃ which corresponds with aprofile p₃′ which the window 75 has, during operation, as a result ofthe deformation of the window 75 by the pressure of the liquid metal inthe constriction 71. Thus, like in the first embodiment shown in FIG. 3,a cross-sectional area of the constriction 71 is substantially constantin the main flow direction X. It is noted, however, that the wall 77 maybe provided with another profile, which matches the profile of thedeformed window 75 in such a manner that during operation a differentpredetermined cross-sectional area, for example a cross-sectional areawhich increases in the main flow direction X like in the embodiment ofFIG. 4, is achieved. Since the profile p₃ of the wall 77 correspondswith or matches the profile p₃′ of the deformed window 75, a center line79 of the constriction 71 is also concave during operation, seen fromthe source 5, so that in the constriction 71 a concave, curved flow ofthe liquid metal is present. As a result, the window 75 is present nearan inner radius of the curved flow, and the wall 77 is present near anouter radius of the curved flow. In this manner it is achieved that thecentrifugal forces, which are exerted on the curved flow of the liquidmetal in the constriction 71, are directed towards the wall 77, i.e.away from the window 75. With, for example, an average flow velocity of50 m/s in the constriction 71, a density of the liquid metal in theorder of 8000 kg/m³, a distance between the window 75 and the wall 77 inthe order of 250 μm, and a curvature of the center line 79 in the orderof 10 mm, an additional pressure in the order of 5 bar is exerted on thewall 77 by the centrifugal forces. In this manner, an additionalmechanical load on the window 75 as a result of said centrifugal forcesis prevented, so that the risk of breakage of the window 75 is furtherreduced.

In the fifth embodiment, shown in FIG. 7, the pressure exerted duringoperation on the window 75 by the liquid metal is below 1 bar,particularly if the cross-sectional area of the constriction 71increases in the main flow direction X, such that the decrease of thepressure in the flow direction X, caused by flow losses, issubstantially compensated by the increase of the pressure in the flowdirection X caused by the Bernoulli effect. However, when the device isstarted or stopped, the pressure on the window 75 may rise well above 1bar. In order to prevent excessive deformations of the window 75,particularly when the device is started or stopped, the window 75 isprovided with a plurality of corrugations 81, as shown in FIG. 8. In theembodiment shown, the corrugations 81 are mutually parallel and extendin the main flow direction X, which is perpendicular to the plane of thedrawing of FIG. 8. In this way irregularities of the flow of liquidmetal at the location of the window 75, such as separations of the flow,are prevented as much as possible, so that the transport of heat awayfrom the impingement position 29 is hardly affected, or not at all, bythe presence of the corrugations 81, and the increase of flow losses inthe constriction 71 as a result of the presence of the corrugations 81is limited. In the embodiment shown, the corrugations 81 each have aheight h of approximately 100 μm and a width w of approximately 100 μm,and between the corrugations 81 a pitch P of approximately 1 mm ispresent. As a result of the presence of the corrugations 81, themechanical stiffness of the window 75 is considerably improved, so thatthe deformation of the window 75 under the influence of the pressure ofthe liquid metal in the constriction 71 is limited, and the risk ofdamage or breakage of the window 75, particularly when the device isstarted or stopped, is further reduced. In the embodiment shown in FIG.8, the wall 77 opposite to the window 75 is provided with a plurality ofcorrugations 83 which correspond with the corrugations 81 provided inthe window 75, i.e. the shape of the corrugations 83 is identical to theshape of the corrugations 81 and the positions of the corrugations 83,seen in a direction Y perpendicular to the main flow direction X, areidentical to the positions of the corrugations 81. In this manner it isachieved that the local distance between the window 75 and the wall 77,i.e. the local height of the constriction 71, is not influenced by thepresence of the corrugations 81. As a result, the presence of thecorrugations 81 does not influence the cross-sectional area of theconstriction 71 and hence does not lead to local deviations of the mainflow velocity and of the pressure of the liquid metal in theconstriction 71.

In the embodiment of FIG. 7 with the concave window 75, the corrugations81 are particularly suitable to prevent buckling of the window 75 underthe influence of the pressure of the liquid metal in the constriction71. It is noted that corrugations similar to the corrugations 81, or analternative structure for increasing the stiffness of the window, mayalso be provided in the embodiments shown in FIGS. 3, 4, 5, and 6 inwhich the window 23 is convex. In these embodiments, buckling of thewindow 23 will not occur as a result of the fact that the window 23 isconvex, but the corrugations can be used here to limit deformation ofthe window 23, particularly stretching of the window 23, under theinfluence of the pressure of the liquid metal. Instead of thecorrugations 81, the window 75 in the embodiment of FIG. 7 may beprovided with an alternative structure to prevent buckling of the window75. FIG. 9 schematically shows an alternative solution to preventbuckling or deformations of the window 75 during starting or stopping ofthe device. This solution may be used in addition to the corrugations 81or instead of the corrugations 81, as the corrugations 81 are mainlynecessary to prevent buckling or deformation of the window 75 duringstarting and stopping of the device. Said solution involves the use of aplug 85 which is provided with a convex surface 87 having a profilecorresponding with the profile of the window 75 when not deformed. Theplug 85 can be positioned such that the convex surface 87 is in contactwith the window 75. For this purpose, the device is provided with anelectro-mechanical positioning device which is not shown in the Figure.The plug 85 is in a position in which it is in contact with the window75 when the device is idle and when the device is started or stopped.When the device is started, the plug 85 will be held in contact with thewindow 75 as long as the pressure of the liquid metal in theconstriction 71 has not yet reached its operational value below 1 bar.When the pressure has reached its operational value, the plug 85 isremoved by means of said positioning device. Before the device isstopped, i.e. before the flow of the liquid metal in the constriction 71is decelerated, the plug 85 is positioned back into contact with thewindow 75 by means of the positioning device.

1. A device for generating X-rays, which device comprises a source foremitting electrons accommodated in a vacuum space, a liquid metal foremitting X-rays as a result of the incidence of electrons, and a pumpingmeans for causing a flow of the liquid metal through a constrictionwhere the electrons emitted by the source impinge upon the liquid metal,said constriction being bounded by a window, which is transparent toelectrons and X-rays and separates the constriction from the vacuumspace, and by a wall opposite to the window, wherein at least duringoperation, said wall has a profile which matches a profile which thewindow has, during operation, as a result of a deformation of the windowcaused by a pressure of the liquid metal in the constriction.
 2. Adevice as claimed in claim 1, wherein said wall is deformable by meansof at least one actuator, the device further comprising at least onepressure sensor for measuring the pressure of the liquid metal in theconstriction and a control member for controlling the actuator as afunction of a pressure measured by means of the sensor.
 3. A device asclaimed in claim 2, wherein said actuator is a piezo-electric actuator.4. A device as claimed in claim 1, wherein in the case of a deformationof the window, during operation, the constriction has a cross-sectionalarea which, seen in a flow direction, increases in such a manner that areduction of the flow velocity in the flow direction takes place suchthat a decrease of the pressure of the liquid metal in the constriction,caused by viscous flow losses, substantially corresponds with anincrease of said pressure caused by said reduction of the flow velocity.5. A device as claimed in claim 1, wherein the device is provided with aflow channel for the liquid metal which successively comprises, seen ina flow direction, a converging part, said constriction, and a divergingpart, wherein a center line of at least a portion of said convergingpart, via which the converging part is connected to the constriction,has a curvature which matches a curvature of a center line which theconstriction has, during operation, in the case of a deformation of thewindow.
 6. A device as claimed in claim 1, wherein the device isprovided with a flow channel for the liquid metal which successivelycomprises, seen in a flow direction, a converging part, saidconstriction, and a diverging part, wherein the converging part isprovided with means for generating or increasing a turbulence of theflow of the liquid metal in the constriction.
 7. A device as claimed inclaim 1, wherein a center line, which the constriction has duringoperation as a result of said deformation of the window, is convex, seenfrom the source.
 8. A device as claimed in claim 1, wherein the windowis concave, seen from the source.
 9. A device as claimed in claim 8,wherein the window is provided with corrugations.
 10. A device asclaimed in claim 9, wherein said corrugations extend in a flow directionof the liquid metal in the constriction.
 11. A device as claimed inclaim 10, wherein the wall opposite to the window is provided withcorrugations which correspond with the corrugations of the window andare in positions, seen in a direction perpendicular to the flowdirection, identical to the positions of the corrugations of the window.